Low-temperature Ground State Research Articles

Emerging single-phase state in small manganite nanodisks

In complex oxides systems such as manganites, electronic phase separation (EPS), a consequence of strong electronic correlations, dictates the exotic electrical and magnetic properties of these materials. A fundamental yet unresolved issue is how EPS responds to spatial confinement; will EPS just scale with size of an object, or will the one of the phases be pinned? Understanding this behavior is critical for future oxides electronics and spintronics because scaling down of the system is unavoidable for these applications. In this work, we use La0.325Pr0.3Ca0.375MnO3 (LPCMO) single crystalline disks to study the effect of spatial confinement on EPS. The EPS state featuring coexistence of ferromagnetic metallic and charge order insulating phases appears to be the low-temperature ground state in bulk, thin films, and large disks, a previously unidentified ground state (i.e., a single ferromagnetic phase state emerges in smaller disks). The critical size is between 500 nm and 800 nm, which is similar to the characteristic length scale of EPS in the LPCMO system. The ability to create a pure ferromagnetic phase in manganite nanodisks is highly desirable for spintronic applications.

Magnetic field-tuned Fermi liquid formation in [formula omitted

Low temperature ground state properties of Mn0.75Fe0.25Si are investigated based on temperature and magnetic field dependent behavior of specific heat and resistivity. Paramagnon spin fluctuations assisted non-Fermi liquid is suppressed by magnetic fields and gradual evolution of Fermi liquid is demonstrated. The tendency of magnetic field-induced crossover from non-Fermi to Fermi liquid behavior is illustrated by suitable magneto-specific heat scaling which reveals unusual quantum critical phenomenon in a 3d transition metal derived paramagnetic compound. In the absence of magnetic fields, Kadowaki–Wood's Ratio (KWR) A/γ2 is about 8.5μΩcmmol2K2J−2 which is 85% close to the originally proposed KWR and reaches 100% under the moderate magnetic fields of 0.7T.

Magnetic properties and specific heat of R2Ni2In (R=Gd-Tm) compounds

The R2Ni2In (R=Gd-Tm) compounds were investigated by means of X-ray diffraction, magnetic and specific heat measurements. The compounds crystallize in the orthorhombic Mn2AlB2-type crystal structure. A low temperature ground state is antiferromagnetic. Critical temperature of magnetic ordering does not fulfill the de Gennes scaling which points to the influence of crystal electric field on stabilization of magnetic order.

3d magnetism in ThCo2Sn2 single crystals

Intense efforts have been made in order to synthesize new tetragonal ternary intermetallic compounds with low-temperature ground states on the border of magnetism. Here we report the synthesis of ThCo2Sn2 single crystals, which crystallize in the tetragonal CaBe2Ge2-type structure, using the flux technique. Further investigation of the physical properties by magnetic susceptibility and pressure-dependent electrical resistivity measurements confirm the presence of antiferromagnetic ordering associated with the Co ions, but with a higher ordering temperature of TN = 78 K when compared to TN = 65 K of the polycrystalline counterpart. Our results show that both Cu-substitution and applied pressure up to 22 kbar are not able to induce superconductivity in this system. Nevertheless, our analyses point out to a putative quantum critical point at higher pressures.

Magnetic properties and hyperfine interactions in Cr₈, Cr₇Cd, and Cr₇Ni molecular rings from ¹⁹F-NMR.

A detailed experimental investigation of the (19)F nuclear magnetic resonance is made on single crystals of the homometallic Cr8 antiferromagnetic molecular ring and heterometallic Cr7Cd and Cr7Ni rings in the low temperature ground state. Since the F(-) ion is located midway between neighboring magnetic metal ions in the ring, the (19)F-NMR spectra yield information about the local electronic spin density and (19)F hyperfine interactions. In Cr8, where the ground state is a singlet with total spin S(T) = 0, the (19)F-NMR spectra at 1.7 K and low external magnetic field display a single narrow line, while when the magnetic field is increased towards the first level crossing field, satellite lines appear in the (19)F-NMR spectrum, indicating a progressive increase in the Boltzmann population of the first excited state S(T) = 1. In the heterometallic rings, Cr7Cd and Cr7Ni, whose ground state is magnetic with S(T) = 3/2 and S(T) = 1/2, respectively, the (19)F-NMR spectrum has a complicated structure which depends on the strength and orientation of the magnetic field, due to both isotropic and anisotropic transferred hyperfine interactions and classical dipolar interactions. From the (19)F-NMR spectra in single crystals we estimated the transferred hyperfine constants for both the F(-)-Ni(2+) and the F(-)-Cd(2+) bonds. The values of the hyperfine constants compare well to the ones known for F(-)-Ni(2+) in KNiF3 and NiF2 and for F(-)-Cr(3+) in K2NaCrF6. The results are discussed in terms of hybridization of the 2s, 2p orbitals of the F(-) ion and the d orbitals of the magnetic ion. Finally, we discuss the implications of our results for the electron-spin decoherence.

Colloquium: The unexpected properties of alkali metal iron selenide superconductors

The iron-based superconductors that contain FeAs layers as the fundamental building block in the crystal structures have been rationalized in the past using ideas based on the Fermi Surface nesting of hole and electron pockets when in the presence of weak Hubbard $U$ interactions. This approach seemed appropriate considering the small values of the magnetic moments in the parent compounds and the clear evidence based on photoemission experiments of the required electron and hole pockets. However, recent results in the context of alkali metal iron selenides, with generic chemical composition $A_x$Fe$_{2-y}$Se$_2$ ($A$ = alkali element), have drastically challenged those previous ideas since at particular compositions $y$ the low-temperature ground states are insulating and display antiferromagnetic magnetic order with large iron magnetic moments. Moreover, angle resolved photoemission studies have revealed the absence of hole pockets at the Fermi level in these materials. The present status of this exciting area of research, with the potential to alter conceptually our understanding of the iron-based superconductors, is here reviewed, covering both experimental and theoretical investigations. Other recent related developments are also briefly reviewed, such as the study of selenide two-leg ladders and the discovery of superconductivity in a single layer of FeSe. The conceptual issues considered established for the alkali metal iron selenides, as well as the several issues that still require further work, are discussed in the text.

The magnetic structure of Co(NCNH)2 as determined by (spin-polarized) neutron diffraction

The magnetic structure of Co(NCNH) 2 has been studied by neutron diffraction data below 10 K using the SPODI and DNS instruments at FRM II, Munich. There is an intensity change in the (1 1 0) and (0 2 0) reflections around 4 K, to be attributed to the onset of a magnetic ordering of the Co 2+ spins. Four different spin orientations have been evaluated on the basis of Rietveld refinements, comprising antiferromagnetic as well as ferromagnetic ordering along all three crystallographic axes. Both residual values and supplementary susceptibility measurements evidence that only a ferromagnetic ordering with all Co 2+ spins parallel to the c axis is a suitable description of the low-temperature magnetic ground state of Co(NCNH) 2 . The deviation of the magnetic moment derived by the Rietveld refinement from the expectancy value may be explained either by an incomplete saturation of the moment at temperatures slightly below the Curie temperature or by a small Jahn–Teller distortion. The magnetic ground state of Co(NCNH) 2 has been clarified by (spin-polarized) neutron diffraction data at low temperatures. Intensity changes below 4 K arise due to the onset of ferromagnetic ordering of the Co 2+ spins parallel to the c axis, corroborated by various (magnetic) Rietveld refinements. • Powderous Co(NCNH) 2 has been subjected to (spin-polarized) neutron diffraction. • Magnetic susceptibility data of Co(NCNH) 2 have been collected. • Below 4 K, the magnetic moments align ferromagnetically with all Co 2+ spins parallel to the c axis. • The magnetic susceptibility data yield an effective magnetic moment of 4.68 and a Weiss constant of −13(2) K. • The ferromagnetic Rietveld refinement leads to a magnetic moment of 2.6 which is close to the expectancy value of 3.

Transient Spin Density Wave Order Induced in the Normal State of BaFe2As2by Coherent Lattice Oscillations

We trace the ultrafast dynamics of the spin density wave gap of the pnictide system BaFe2As2 probing resonantly with broadband multi-terahertz pulses. The photoexcitation in the low-temperature ground state leads to a fast suppression of the spin order followed by a slower recovery process. Surprisingly, in the normal state, we observe periodic oscillations of the spin density wave feature at a frequency as high as 5.5 THz. Our results indicate a transient development of a macroscopic order driven by a coherent lattice oscillation and attest to a pronounced spin–phonon coupling in pnictides.

Low-temperature magnetization in geometrically frustrated Tb2Ti2O7

The nature of the low temperature ground state of the pyrochlore compound Tb2Ti2O7 remains a puzzling issue. Dynamic fluctuations and short-range correlations persist down to 50 mK, as evidenced by microscopic probes. In parallel, magnetization measurements show irreversibilities and glassy behavior below 200 mK. We have performed magnetization and AC susceptibility measurements on four single crystals down to 57 mK. We did not observe a clear plateau in the magnetization as a function of field along the [111] direction, as suggested by the quantum spin ice model. In addition to a freezing around 200 mK, slow dynamics are observed in the AC susceptibility up to 4 K. The overall frequency dependence cannot be described by a canonical spin-glass behavior.

Neutron scattering from the static and dynamic lattice of SrCu2(BO3)2in its Shastry-Sutherland singlet ground state

Elastic and inelastic neutron scattering results show that SrCu${}_{2}$(BO${}_{3}$)${}_{2}$ enters its low-temperature singlet ground state below 10 K without an obvious accompanying structural phase transition, despite suggestions emanating from earlier heat capacity measurements. However, evidence for significant spin-phonon coupling is found in the energy widths, and the corresponding lifetimes, of transverse acoustic phonons propagating in the ($H$00) direction of the Shastry-Sutherland, tetragonal basal plane. Transverse acoustic phonons with energies comparable to and higher than the onset of the two-triplet continuum show substantially increased lifetimes on entering the singlet ground state below $\ensuremath{\sim}$10 K. This is qualitatively consistent with the removal of a decay channel for the phonons due to the gapping of the spin excitation spectrum in SrCu${}_{2}$(BO${}_{3}$)${}_{2}$ at low temperatures.

Ultrafast transient generation of spin-density-wave order in the normal state of BaFe2As2 driven by coherent lattice vibrations

The interplay among charge, spin and lattice degrees of freedom in solids gives rise to intriguing macroscopic quantum phenomena such as colossal magnetoresistance, multiferroicity and high-temperature superconductivity1,2,3. Strong coupling or competition between various orders in these systems presents the key to manipulate their functional properties by means of external perturbations such as electric and magnetic fields2 or pressure3. Ultrashort and intense optical pulses have emerged as an interesting tool to investigate elementary dynamics and control material properties by melting an existing order4,5,6. Here, we employ few-cycle multi-terahertz pulses to resonantly probe the evolution of the spin-density-wave (SDW) gap of the pnictide compound BaFe2As2 following excitation with a femtosecond optical pulse. When starting in the low-temperature ground state, optical excitation results in a melting of the SDW order, followed by ultrafast recovery. In contrast, the SDW gap is induced when we excite the normal state above the transition temperature. Very surprisingly, the transient ordering quasi-adiabatically follows a coherent lattice oscillation at a frequency as high as 5.5 THz. Our results attest to a pronounced spin–phonon coupling in pnictides that supports rapid development of a macroscopic order on small vibrational displacement even without breaking the symmetry of the crystal.

Component-specific electromechanical response in a ferroelectric/dielectric superlattice

The electronic and electromechanical properties of complex oxide superlattices are closely linked to the evolution of the structure and electrical polarization of the component layers in applied electric fields. Efforts to deduce the responses of the individual components of the superlattice to applied fields have focused on theoretical approaches because of the limitations of available experimental techniques. Time-resolved x-ray microdiffraction provides a precise crystallographic probe of each component using the shift in wave vector and change in intensity of superlattice satellite reflections. We report in detail the methods to measure and analyze the x-ray diffraction patterns in applied electric field and their application to a 2-unit-cell ${\text{BaTiO}}_{3}/4$-unit-cell ${\text{CaTiO}}_{3}$ superlattice. We find that the overall piezoelectric distortion is shared between the two components. Theoretical predictions of the electromechanical properties of a superlattice with the same composition constrained to tetragonal symmetry are in excellent agreement with the experiments. Lattice instability analysis, however, suggests that the low-temperature ground state could exhibit antiferrodistortive rotations of ${\text{TiO}}_{6}$ octahedra within and/or at the interfaces of the ${\text{CaTiO}}_{3}$ component.

Topological insulators and metal-insulator transition in the pyrochlore iridates

The possible existence of topological insulators in cubic pyrochlore iridates A$_{2}$Ir$_{2}$O$_{7}$ (A = Y or rare-earth elements) is investigated by taking into account the strong spin-orbit coupling and trigonal crystal field effect. It is found that the trigonal crystal field effect, which is always present in real systems, may destabilize the topological insulator proposed for the ideal cubic crystal field, leading to a metallic ground state. Thus the trigonal crystal field is an important control parameter for the metal-insulator changeover. We propose that this could be one of the reasons why distinct low temperature ground states may arise for the pyrochlore iridates with different A-site ions. On the other hand, examining the electron-lattice coupling, we find that softening of the $\textbf{q}$=0 modes corresponding to trigonal or tetragonal distortions of the Ir pyrochlore lattice leads to the resurrection of the strong topological insulator. Thus, in principle, a finite temperature transition to a low-temperature topological insulator can occur via structural changes. We also suggest that the application of the external pressure along [111] or its equivalent directions would be the most efficient way of generating strong topological insulators in pyrochlore iridates.

Effect of Bi doping on magnetic and magnetocaloric properties of La0.7−xBixSr0.3MnO3 (≤x≤0.4)

We investigated the effect of Bi doping on magnetic and magnetocaloric properties of La0.7−xBixSr0.3MnO3 (x=0.0–0.4). It is shown that the low temperature ground state changes from a ferromagnet (x=0) to a charge-ordered antiferromagnet for x=0.4. While the paramagnetic-ferromagnetic (PM-FM) transition is second-order in x≤0.25, it changes into first-order for x=0.3 which is at the magnetic phase boundary. The changes in the magnetic ground state affect magnetic entropy. The magnitude of the isothermal magnetic entropy (|ΔSM|) at the FM Curie temperature increases from 4.56 J/kg K for x=0 to a maximum value of 5.02 J/kg K for x=0.05 and then decreases to nearly zero for x=0.4 at the charge order transition. In contrast to x≤0.25, the ΔSM of x=0.3 is magnetic history dependent and its temperature dependence exhibits a clear step at TCO=260 K followed by a plateau between 240 and 185 K. Although |ΔSM|=3.1 J/kg K of x=0.3 is small compared to other compositions, it has a high relative cooling power (325 J/kg) which is desirable for magnetic refrigeration over a wide temperature. The unusual magnetic and magnetocaloric properties of x=0.3 are attributed to the existence of short-range charge-orbital (CO) correlations in the PM state. It is suggested that harnessing competition between FM spin ordering and CO correlations may provide a strategy to enhance magnetic refrigeration capacity over a wide temperature range.

First-principles determination of low-temperature order and ground states of Fe-Ni, Fe-Pd, and Fe-Pt

While the binary $\text{Fe-}X$ $(X=\text{Ni},\text{Pd},\text{Pt})$ alloys are among the most widely applied bimetallics, open questions remain regarding whether and which of their compounds are stable at low $T$. Based on density-functional theory and first-principles cluster expansions that are ``filtered'' against structural and magnetic bistabilities, we assess all three systems. We (i) review the stability of the known phases; (ii) predict phases unstable with respect to bcc-fcc mixtures but stable if restricted to fcc; and, (iii) remarkably, predict previously unknown stable phases. This pinpoints where more definitive low-$T$ experiments should find new stable compounds.

Variational study of the antiferromagnetic insulating phase ofV2O3based onNthorder muffin-tin-orbitals

Motivated by recent results of $N\text{th}$ order muffin-tin-orbital (NMTO) implementation of density-functional theory, we re-examine low-temperature ground-state properties of the antiferromagnetic insulating phase of vanadium sesquioxide ${\text{V}}_{2}{\text{O}}_{3}$. In fact, the hopping matrix elements within the nearest-neighbor vanadium pair, obtained by the NMTO-downfolding procedure, are strongly reduced compared to those previously obtained using the downfolding procedure of Castellani et al. [Phys. Rev. B18, 4945 (1978); 18, 4967 (1978);18, 5001 (1978)]. This could imply a breakdown of the molecular picture. We use the NMTO hopping matrix elements as input and perform a variational study of the ground state. We find that the formation of stable molecules throughout the crystal is not favorable in this case, though the experimentally observed magnetic structure can still be obtained in the atomic variational regime. However, the resulting ground state (two ${t}_{2g}$ electrons occupying the degenerate ${e}_{g}$ doublet) is in contrast with many well-established experimental observations. We discuss the implications of this finding in the light of the importance of nonlocal electronic correlations in ${\text{V}}_{2}{\text{O}}_{3}$.

Calculation of solubility in titanium alloys from first principles

We present an approach to calculate the atomic bulk solubility in binary alloys based on the statistical-thermodynamic theory of dilute lattice gas. The model considers all the appropriate ground states of the alloy and results in a simple Arrhenius-type temperature dependence determined by a “low-solubility formation enthalpy”. This quantity, directly obtainable from first principles, is defined as the composition derivative of the compound formation enthalpy with respect to nearby ground states. We apply the framework and calculate the solubility of the A solutes in A–Ti alloys (A = Ag, Au, Cd, Co, Cr, Ir, W, Zn). In addition to determination of unknown low-temperature ground states for the eight alloys, we find qualitative agreements with solubility experimental results. The presented formalism, correct in the low-solubility limit, should be considered as an appropriate starting point for estimation of whether a more computationally expensive formalism is needed.

Quantum oscillations in the parent pnictideBaFe2As2: Itinerant electrons in the reconstructed state

We report quantum-oscillation measurements that enable the direct observation of the Fermi surface of the low-temperature ground state of ${\text{BaFe}}_{2}{\text{As}}_{2}$. From these measurements we characterize the low-energy excitations, revealing that the Fermi surface is reconstructed in the antiferromagnetic state, but leaving itinerant electrons in its wake. The present measurements are consistent with a conventional band folding picture of the antiferromagnetic ground state, placing important limits on the topology and size of the Fermi surface.

Low temperature properties of a new compound based on (Yb, Zr)B12 substitution

The thermodynamic, kinetic, and magnetic characteristics of (Yb, Zr)B12 have been investigated in detail in the temperature range from 4 to 300 K to reveal the effect of the band structure on the properties of the YbB12 Kondo insulator ground state. It is found that electron doping due to 20% substitution in the Yb sublattice significantly changes the properties of the low-temperature ground state of YbB12 Kondo insulator and only slightly affects the high-temperature spin-fluctuating state forming at T > 50 K.

A study of the electronic structure of the first-row transition-metal compounds

A study has been made of the electronic structure of the first-row transition-metal compounds. In this study, particular attention has been given to the properties of the first-row transition-metal mono-oxides and a brief discussion of these properties is presented. The inability of either conventional energy band theory or of the ligand or crystal field theories to adequately describe the electronic structure of these compounds is brought out in this discussion. It is found that a model based upon the spin polarized energy band theory of solids, which allows for the localized or crystal field-like transitions that are characteristic of many of these compounds, is capable of adequately describing their electronic structure. The spin polarized energy band structures of MnO, α-MnS, and NiO are presented and discussed. These calculations lead to the prediction that MnO and α-MnS should be insulators and that NiO should be a semiconductor. It is shown that this nonmetallic behavior should continue after the loss of the antiferromagnetic ordering that characterizes their low temperature ground states. The importance of the intra-atomic exchange contributions to these band structures is discussed. A discussion is also given of the form of the exchange potential that is being used in conducting these studies.